Transformer and switched-mode power supply apparatus

ABSTRACT

A transformer is capable of suppressing the output voltage difference, and a switched-mode power supply apparatus uses the transformer. A transformer has a core; a primary winding provided in the core; a gap provided in the core at a location where the primary winding is provided; and at least two secondary windings, provided in the core and spaced apart from both sides of the primary winding as well as the gap at an equal distance in a winding axis direction of the primary winding. A switched-mode power supply apparatus has the transformer; a switching element connected to the primary winding of the transformer; and a control circuit configured to control the switching element.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a transformer and a switched-mode power supply apparatus using the same, particularly a configuration of the transformer.

2. Background of the Invention

In order to operate an electrical device or electrical circuit, stable DC voltage is required and a switched-mode power supply apparatus has been conventionally used for this purpose. The switched-mode power supply apparatus rectifies and smoothes a voltage which is supplied by an external power source; performs switching operation due to a semiconductor switching element to input the voltage into a primary winding of the transformer; and supplies DC voltage from a secondary winding to a load through a smoothing operation performed by a smoothing circuit. Furthermore, an output voltage is monitored, and the time ratio of the semiconductor switching element is adjusted by a control circuit so that the output voltage is constantly maintained.

As the switched-mode power supply apparatus, multi-output switched-mode power supply apparatus has been conventionally known. A transformer for the above multi-output switched-mode power supply apparatus has a core, a primary winding which is provided in a core, and at least two secondary windings which are also provided in the core.

CITATION LIST Patent Literature

-   [PLT 1] Japanese Publication 05-049257(A)

SUMMARY OF THE INVENTION Problem to be Solved

In such a multi-output transformer in which at least two secondary windings are provided, a distance from a gap which is provided in the core to each of the secondary windings is different from each other, which renders the inductance of each secondary winding different. Such a conventional multi-output transformer is provided as a cross-sectional view in FIG. 14. Referring to FIG. 14, the secondary windings 112, 113 which are disposed at both sides of the primary winding 111 are asymmetrically disposed with respect to the gap 115 which is provided in the core 110. In other words, the secondary windings 112, 113 are not spaced apart from the gap 115 at an equal distance. In this case, due to the different distance between each of the secondary winding 112, 113 and the gap 115, the inductances become different. For the above reasons, the output voltages from the secondary windings 112, 113 differ from each other.

Furthermore, even if each of the secondary windings 112, 113 is spaced apart from the gap 115 at an equal distance, as shown in FIG. 16, due to the distance difference between each of the secondary winding 112, 113 and the primary winding 111, the coupling coefficient of the secondary windings 112, 113 with respect to the primary winding 111 become different from each other. For the above reasons, the output voltages from the secondary windings 112, 113 differ from each other.

The invention has been made for solving the above problems or drawbacks, and provides a transformer being capable of suppressing the output voltage difference, and a switched-mode power supply apparatus using the transformer.

Solution to Solve the Problem

(1) An embodiment of the transformer has a core; a primary winding provided in the core; a gap provided in the core at a location where the primary winding is provided; and at least two secondary windings provided in the core and spaced apart from both sides of the primary winding as well as the gap at an equal distance in a winding axis direction of the primary winding.

The embodiment of the transformer may have the following configuration.

(2) The transformer may further have at least two auxiliary windings provided in the core. The auxiliary windings may respectively neighbor the secondary windings in the winding axis direction of the primary winding, and be connected in parallel to each other.

(3) The transformer may further have at least two auxiliary windings provided in the core. The secondary windings may be disposed closer to the primary winding than the auxiliary windings.

(4) An embodiment of a switched-mode power supply apparatus has the transformer in accordance with the above (1); a switching element connected to the primary winding of the transformer; and

a control circuit configured to control the switching element. Moreover, the transformer may have the configuration of the above (2) or (3).

Effect of the Invention

In accordance with the invention, the transformer in which the inductance difference and the difference of the coupling coefficient with respect to the primary winding between the two secondary windings are decreased to suppress the output voltage difference between the secondary windings, and the switched-mode power supply apparatus using the transformer can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing an entire configuration of a first embodiment of a transformer.

FIG. 2 is a circuit diagram of a switched-mode power supply apparatus provided with the first embodiment of the transformer.

FIG. 3 is a graph showing an output voltage from each of secondary windings of the first embodiment of the transformer in the elapsed time.

FIG. 4 is a schematic cross-sectional view showing an entire configuration of the second embodiment of a transformer.

FIG. 5 is a circuit diagram of a switched-mode power supply apparatus provided with the second embodiment of the transformer.

FIG. 6 is a graph showing an output voltage (after rectification) from each of secondary windings of the first embodiment of the transformer with a changed load balance in the elapsed time.

FIG. 7 is a graph showing an output voltage (after rectification) from each of secondary windings of the second embodiment of the transformer with a changed load balance in the elapsed time.

FIG. 8A shows an output voltage waveform (before rectification) of a secondary winding in a case where a current flowing through a secondary winding 12 is 0 A (i.e., no load) and a current flowing through a secondary winding 13 is 0.1 A.

FIG. 8B shows an output voltage waveform (before rectification) of a secondary winding in a case where a current flowing through a secondary winding 12 is 0.1 A and a current flowing through a secondary winding 13 is 0 A (i.e., no load).

FIG. 9A is a graph showing a voltage of an auxiliary winding of the first embodiment of the transformer with a changed load balance in the elapsed time in a case where a current flowing through one of the secondary windings is 0.1 A and a current flowing through the other of the secondary windings is 0 A.

FIG. 9B is a graph showing a voltage of an auxiliary winding of the first embodiment of the transformer with a changed load balance in the elapsed time in a case where a current flowing through one of the secondary windings is 0 A and a current flowing through the other of the secondary windings is 0.1 A.

FIG. 10 is a graph showing a voltage of each of auxiliary windings of the second embodiment of the transformer in the elapsed time.

FIG. 11 shows a voltage waveform (before rectification) in a case where a current flowing through a secondary winding 12 is 0 A and a current flowing through a secondary winding 13 is 0.1 A.

FIG. 12 is a schematic cross-sectional view showing an entire configuration of another embodiment of a transformer.

FIG. 13 is a circuit diagram of a switched-mode power supply apparatus provided with another embodiment of the transformer.

FIG. 14 is a schematic cross-sectional view showing an entire configuration of a conventional transformer.

FIG. 15 is a graph showing an output voltage from each of secondary windings of the conventional transformer in the elapsed time.

FIG. 16 is a schematic cross-sectional view showing an entire configuration of a conventional transformer.

FIG. 17 is a graph showing an output voltage from each of secondary windings of the conventional transformer in the elapsed time.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the accompanying drawings, an embodiment of a transformer in accordance with the invention and a switched-mode power supply apparatus provided with the transformer will be hereinafter described. The embodiment of the transformer is firstly described and the configuration of the switched-mode power supply apparatus using the transformer is described later.

1. First Embodiment [1-1. Configuration]

FIG. 1 is a schematic cross-sectional view of an entire configuration of the first embodiment of a transformer in accordance with the invention. A transformer T transforms a voltage which is supplied by an external power source, and provides electric power to a load such as an external device or a circuit connected to the transformer T. Referring to FIG. 1, the transformer T has a core 10, a winding 11 provided in the core 10, at least two secondary windings 12, 13 (in the embodiment, two secondary windings), and an auxiliary winding 14.

The core 10 is a magnetic body such as a ferrite magnetic core, a laminated steel plate and a compressed powder magnetic core. While in the embodiment the core 10 has a cross-section of approximately θ shape, the shape of the core 10 is not limited thereto. This is because it would be enough that the primary winding 11, the secondary windings 12, 13, and the auxiliary winding 14 can be mounted or provided thereto.

The core 10 has a linear center leg portion 10 a in the center portion thereof. The center log portion 10 a is provided with the windings 11-14 such that the winding axis of the windings 11-14 is arranged on the same line C. A gap 15 is provided in the core 10 at a location where the primary winding 11 is provided. In other words, the gap 15 is provided in the center leg portion 10 a. Furthermore, the secondary windings 12, 13, and the auxiliary winding 14 are wound around the center leg portion 10 a in a state the polarity thereof is reversed with respect to the polarity of the primary winding 11. The core 10, the windings 11-14 are respectively insulated by a bobbin (not shown) formed of insulating material such as resin.

The primary winding 11 is connected to an external power source, and supplies electric power to the secondary windings 12, 13 and the auxiliary winding 14. The secondary windings 12, 13 are connected to the load such as the external device or the external circuit between both terminals, and supply electric power which is supplied by the primary winding 11 to the circuit or the load. The secondary windings 12, 13 are connected to, for example, a buffer circuit for operating IGBT, MOS, and the like.

The secondary windings 12, 13 are disposed at both sides of the primary winding 11 in a winding axis of the primary winding 11. In other words, both of the secondary windings 12 and 13 are disposed adjacent to the primary winding 11. Preferably, the secondary windings 12 and 13 are respectively spaced apart from the primary winding 11 at an equal distance in the winding axis of the primary winding 11. Furthermore, the secondary windings 12, 13 are respectively spaced apart from the gap 15 at an equal distance in the winding axis of the primary winding 11.

In this embodiment, the secondary windings 12, 13 are respectively spaced apart from the primary winding 11 as well as the gap 15 at an equal distance in the winding axis of the primary winding 11. In other words, the secondary windings 12, 13 are symmetrically arranged with respect to the primary winding 11, and are symmetrically arranged with respect to the gap 15.

The auxiliary winding 14 is connected to a control circuit for controlling a switching element which is described below. The auxiliary winding 14 receives electric power from the primary winding 11 to provide electric power voltage for driving the control circuit. While the auxiliary winding 14 is disposed next to the secondary winding 13 in the same winding axis as the secondary winding 13, it may be disposed next to the secondary winding 12.

Such a transformer T can be used in a switched-mode power supply apparatus. FIG. 2 is a circuit diagram of a switched-mode power supply apparatus provided with the first embodiment of the transformer. Since the transformer T is provided with a plurality of the secondary windings 12, 13 (in the embodiment, two secondary windings), the switched-mode power supply apparatus provided with the embodiment of the transformer (i.e., the transformer T) corresponds to a multi-output power supply apparatus. The switched-mode power supply apparatus is, for example, a flyback switched-mode power supply apparatus, and used for switching a semiconductor.

Specifically, the switched-mode power supply apparatus has the transformer T, a switching element 21, a control circuit 22 for controlling the switching element 21, diodes 23, 24, and capacitors 25, 26. The switched-mode power supply apparatus may have a rectifying/smoothing circuit. The rectifying/smoothing circuit is connected between the external power source and the primary winding 11 of the transformer T to rectify and smooth the voltage supplied by the external power source.

The switching element 21 is a semiconductor switching element such as FET. The switching element 21 is connected to the primary winding 11 of the transformer T to control the input voltage into the primary winding 11. The control circuit 22 is equipped with IC, and connected to the switching element 21 and the auxiliary winding 14 which is provided at the output side. The control circuit 22 receives the power voltage supply from the auxiliary winding 14 to control the time ratio of on/off of the switching element 21 for the purpose of controlling the input voltage into the primary winding 11. In other words, the control circuit 22 performs a control for the purpose of keeping the output voltage of the secondary windings 12 and 13 at a predetermined voltage.

For example, the control circuit 22 may have voltage monitoring means for detecting the voltage of the auxiliary winding 14, smoothing means such as a capacitor for smoothing the output voltage from the auxiliary winding 14, a photocoupler provided with a light-emitting element and a light-receiving element, and IC. In this case, as an exemplary control performed by the control circuit 22, an output voltage value from the auxiliary winding 14 which is smoothed by the smoothing means and detected by the voltage monitor means is firstly input into the IC. The IC calculates the output voltage of the secondary windings 12, 13 based on the (output) voltage value, and the winding number ratio of the auxiliary winding 14 and the secondary windings 12 and 13, and generates the control signal for stabilizing the output voltage of the secondary windings 12, 13 based on the output voltage of the secondary windings 12, 13. The IC outputs the control signal at the light-emitting element of the photocoupler which is connected to the IC. The light-emitting element converts the input control signal into optical signal, and outputs the optical signal at the light-receiving element which is connected to the switching element 21. Furthermore, the light-receiving element converts the input optical signal into electric signal, and changes the time ratio of the switching element 21 based on the electric signal.

The capacitors 25, 26 are connected to the secondary windings 12, 13. The diodes 23, 24 are connected between the secondary windings 12, 13 and the capacitors 25, 26 to rectify the output voltage from the secondary windings 12 and 13. Furthermore, the capacitors 25, 26 smooth the rectified voltage and generate DC voltage.

[1-2. Action and Effect]

(1) The embodiment of the transformer T has the core 10, the primary winding 11 provided in the core 10, the gap 15 provided in the core 10 at the location where the primary winding 11 is provided, and the at least two secondary windings 12, 13 which are provided in the core 10 and spaced apart from the both sides of the primary winding 11 as well as the gap 15 at an equal windings 12, 13 in the winding axis of the primary winding 11. Due to the above configuration, the inductance difference as well as the difference of coupling coefficient with respect to the primary winding 11 between the two secondary windings 12, 13 can be reduced, thereby suppressing the difference between the output voltages of the two secondary windings 12, 13 when the output voltages of the two secondary windings 12, 13 are stabilized, as shown in FIG. 3.

More specifically, the effect of the embodiment is explained in comparison to the conventional technologies. In the conventional transformer in which the secondary windings 112, 113 that are disposed at both sides of the primary winding 111 are not spaced apart from the gap 115 at an equal distance, if the inductance values of the secondary windings 112, 113 are two-figure (digit) μH and the inductance value difference between the secondary windings 112, 113 is single-figure (digit) μH, the output voltages of the second windings differ from each other, as shown in FIG. 15. This is because the difference between the inductance values of the secondary windings 112, 113 is great. In an example as shown in FIG. 14, the inductance values are equalized to only the level of two figure (digit). To the contrary, the inductance values of the secondary windings 12 and 13 of the transformer T (i.e., the embodiment of the transformer) are equalized to the level of the first decimal place. That is, there is two-figure (digit) difference between the conventional technologies and the embodiment of the transformer in terms of the correspondence of the inductance values.

Furthermore, as shown in FIG. 16, in a case of the conventional transformer in which the secondary windings 112, 113 are spaced apart from the gap 115 at an equal distance but not spaced apart from the primary winding 111 at an equal distance, if the inductance values of the secondary windings 112, 113 are two-figure μH and the inductance value difference between the secondary windings 112, 113 is the first decimal place (μH), the output voltages of the second windings 112, 113 differ from each other, as shown in FIG. 17. This is because the inductance values of the secondary windings 112, 113 are almost equal but there is a difference of the coupling with the primary winding 111 between the secondary windings 112, 113. In an example as shown in FIG. 16, the inductance values are equalized to only the level of single figure (digit). Furthermore, the coupling coefficient values of the secondary windings 112, 113 with respect to the primary winding 111 are equalized to only the level of the first decimal place. On the other hand, the inductance values of the secondary windings 12 and 13 of the transformer T (i.e., the embodiment of the transformer) are equalized to the level of the first decimal place. That is, there is single figure (digit) difference between the conventional technologies and the embodiment of the transformer in terms of the correspondence of the inductance values. Furthermore, in the transformer T (i.e., the embodiment of the transformer) the coupling coefficient values with respect to the primary winding 11 are equalized to the level of the second decimal place. That is, there is single figure (digit) difference between the conventional technologies and the transformer T (i.e., the embodiment of the transformer) in terms of the correspondence of the coupling coefficient values.

As described previously, in accordance with the embodiment, the difference of the coupling coefficient with respect to the primary winding 11 and the inductance difference between the secondary windings 12, 13 can be reduced. Therefore, the transformer being capable of synergistically suppressing the difference between the output voltages from the secondary windings 12, 13 and the switched-mode power supply apparatus using the same transformer can be obtained.

2. Second Embodiment [2-1. Configuration]

The second embodiment is described with reference to FIGS. 4-11. The configuration of the second embodiment is basically equal to that of the first embodiment. Therefore, only the difference between the second embodiment and the first embodiment will be described. The same part or portion is denoted by the same reference numeral, and detailed description thereof is omitted.

FIG. 4 is a schematic cross-sectional view showing an entire configuration of the second embodiment of a transformer. FIG. 5 is a circuit diagram of a switched-mode power supply apparatus provided with the second embodiment of the transformer. The second embodiment is different from the first embodiment in that at least two auxiliary windings 14, 16 (two auxiliary windings in the embodiment) are provided.

In other words, the second embodiment is different from the first embodiment in that each of the auxiliary windings 14, 16 neighbors respectively each of the secondary windings 12, 13 in the winding axis direction of the windings 11-13, and connected in parallel to each other. Each of the auxiliary winding 14 and 16 is spaced apart from the gap 15 at an equal distance and arranged symmetrically with respect to the gap 15, in the winding axis direction. However, the auxiliary windings 14 and 16 may not be necessarily arranged symmetrically with respect to the gap 15.

Furthermore, in the embodiment the secondary windings 12, 13 are arranged closer to the primary winding 11 than the auxiliary windings 14, 16. Each of the windings 11-16 is insulated by the bobbin that is formed of insulating material such as resin. Furthermore, as shown in FIG. 5, the auxiliary windings 14, 16 are connected in parallel to the control circuit 22.

[2-2. Action and Effect]

(1) The action and effect of the embodiment (i.e., the second embodiment) will be explained in comparison to that of the first embodiment. In the second embodiment, even in a case where the load is unbalanced (for example, a case where two different loads are respectively connected to the secondary windings 12, 13), variation in the output voltage of the secondary windings 12, 13 can be suppressed. Furthermore, the configuration of the second embodiment can be applied to any transformer which is provided with the primary winding and two or more secondary windings.

Firstly, FIG. 6 shows the waveform of the output voltage of each of the secondary windings 12, 13 in a case where the balance of the load is changed in the configuration of the first embodiment. Specifically, FIG. 6 is an example of an output voltage waveform (after rectification) in a case where the current of the secondary winding 12 is 0 A and the current of the secondary winding 13 is 0.1 A. The width between the dotted lines corresponds to the width between the maximum output voltage of the secondary winding 12 and the minimum output voltage of the secondary winding 13. It can be seen that the gap or interval is about 4.25 V.

On the other hand, the second embodiment has two or more auxiliary windings 14, 16, which in the winding axis direction of the primary winding 11, respectively neighbor the secondary windings 12, 13, and are connected in parallel to each other. Due to this configuration, even in a case where the load becomes unbalanced, cross regulation can be improved. For example, FIG. 7 shows an output voltage waveform of the secondary windings 12, 13 (after rectification) when the balance of the load is changed. The condition of FIG. 7 is similar to that of FIG. 6 in that the current of the secondary winding 12 is 0 A and the current of the secondary winding 13 is 0.1 A. As shown in FIG. 7, the width between the dot-and-dash lines of the maximum output voltage and the minimum output voltage of the secondary windings 12, 13 is about 2.25V, which is less than the width (about 4.25V) between the dotted lines as shown in FIG. 6 and means that variation in the output voltage is less. In other words, it can be seen that the stability of the output voltage is improved, and the cross regulation is improved.

The reason for above phenomenon or improvement can be explained in view of the first embodiment. In the first embodiment, when the load becomes unbalanced, a distortion (i.e., a turn) is created in the output voltage waveform of the secondary winding at the no-load side. An example is shown in FIGS. 8A and 8B. FIGS. 8A and 8B show the output voltage waveform of secondary winding before rectification. FIG. 8A shows the output voltage waveform in a case where a current flowing through the secondary winding 12 is 0 A (no load) and a current flowing through the secondary winding 13 is 0.1 A. FIG. 8B shows the output voltage waveform in a case where a current flowing through the secondary winding 12 is 0.1 A and a current flowing through the secondary winding 13 is 0 A (no load). As such, the distortion in the output voltage waveform at the no-load side means corresponding or proportional variation (change) in the output voltage.

Furthermore, in a case where the distortion is created in the output voltage waveform of the secondary winding, the distortion is also created in the voltage waveform of the auxiliary winding 14 neighboring the above secondary winding. For example, the voltage waveform of the auxiliary winding 14 as shown in FIG. 9B in a case where the current flowing through the secondary winding 12 that is disposed away from the auxiliary winding 14 is 0.1 A and the current flowing through the secondary winding 13 that is disposed adjacent to the auxiliary winding 14 is 0 A is different from the voltage waveform of the auxiliary winding 14 as shown in FIG. 9A in a case where the current flowing through the secondary winding 12 that is disposed away from the auxiliary winding 14 is 0 A and the current flowing through the secondary winding 13 that is disposed adjacent to the auxiliary winding 14 is 0.1 A in the range of from 260 to 262 μs (time), and the distortion is created in FIG. 9B. Since the auxiliary winding 14 is arranged away from the secondary winding 12 and adjacent to the secondary winding 13, the coupling coefficients between the auxiliary winding 14 and each of the secondary windings 12, 13 are different from each other. For the above reasons, the ways of action through the magnetic field of the auxiliary winding 14 on each of the secondary windings 12, 13 are different, and the cross regulation is thus less likely to be improved.

On the other hand, in the embodiment, the two auxiliary windings 14, 16 are provided in the core 10 and connected in parallel to each other. Accordingly, the auxiliary windings 14, 16 is shorted to each other, and the waveforms of the auxiliary windings 14, 16 are equalized. For example, FIG. 10 shows the voltage waveforms of the auxiliary windings 14, 16 in a case where the current of the secondary winding is 0 A and the current of the secondary winding 13 is 0.1 A. It can be seen that the two voltage waveforms are made same. Furthermore, since the auxiliary winding 14 neighbors the secondary winding 13 and the auxiliary winding 16 neighbors the secondary winding 12, the magnetic field generated by each of the auxiliary winding 14, 16 respectively exerts its action on each of the secondary windings 13, 12 which neighbors each of the auxiliary windings 14, 16.

In other words, when the load becomes unbalanced, the voltage waveform of one of the secondary windings 12, 13 is normal and the distortion occurs in the voltage waveform of the other of the secondary windings 12, 13. Along with this, due to the distorted voltage waveform of the secondary winding 12, 13 the voltage waveform of the auxiliary winding 14, 16 neighboring the secondary winding 12, 13 having the distorted voltage waveform also becomes distorted. However, the voltage waveform of the auxiliary winding 14, 16 which neighbors the secondary winding 12, 13 having the normal voltage waveform is normal, and the two auxiliary windings 14, 16 are shorted to each other. As a result, the voltage waveform of the auxiliary winding 14 16 which neighbors the secondary winding 12, 14 having the distorted voltage waveform is normalized. The auxiliary windings 14, 16 having the normalized voltage waveform exerts its action on the secondary winding 12, 13 having the distorted voltage waveform, thereby rendering the distorted voltage waveform normal. As such, the distortion is alleviated. In one example, FIG. 11 shows the voltage waveform (before rectification) in a case where the current of the secondary winding 12 is 0 A and the current of the secondary winding 13 is 0.1 A. As shown by the circular dotted line in FIG. 11, the projection (about 20V) in the voltage waveform of the secondary winding 12 (FIG. 11) is further lowered in comparison to the projection (about 21V) in the voltage waveform of the secondary winding 12 as shown in FIG. 8A. Therefore, it can be seen that the distortion is alleviated. As described above, the voltage waveforms of the secondary windings 12, 13 (before rectification) are improved such that they are equalized, the cross regulation can be reduced.

(2) In the embodiment, the secondary windings 12, 13 are arranged closer to the primary winding 11 than the auxiliary winding 14, 16. Due to this, the coupling coefficient between each of the secondary windings 12, 13 and the primary winding 11 can be increased, thereby improving the transformation (conversion) efficiency of the transformer.

3. Other Embodiments

The invention is not limited to above embodiments, and also encompasses other embodiments described below. Furthermore, by properly combining a plurality of elements described in the above embodiments and other embodiments described below, various modification can be made. During the implementation of the invention, the element can be modified and embodied without departing from the scope of the invention. For example, one or more element may be removed, modified or replaced in the configuration of the embodiments. An exemplary other embodiments are provided below. Furthermore, the switched-mode power supply apparatus using the transformer in accordance with the above embodiments, or the other embodiments described below and the transformer obtained by combining the above embodiment and the other embodiment described below are also included in the scope of the invention.

(1) While the two secondary windings 12, 13 are provided in the first and second embodiments, three or more secondary windings may be provided. In a case where odd numbers (for example, three) of secondary windings are adopted, the two of the secondary windings 12, 13 are symmetrically arranged with respect to the gap 15, and other secondary winding 17 is disposed over either of the secondary windings 12, 13, as shown in FIG. 12. In other word, the secondary winding 17 is provided outside either of the secondary windings 12, 13 such that it is wound around the same winding axis as either of the secondary windings 12, 13 and has a radius that is different from the radius of the either of the secondary windings 12, 13. In this case, the windings are insulated from each other by the resin bobbin and the like.

In a case where the even numbers of the secondary windings are adopted, they are spaced apart from the gap at an equal distance. If four secondary windings are provided, two of the secondary windings are respectively disposed over other two secondary windings, as described previously in connection with the case where odd numbers of secondary windings are adopted. As such, even in the case of multi-output form where three or more secondary windings are adopted, the secondary windings are symmetrically arranged with respect to the gap 15 and spaced apart from the primary winding 11 at an equal distance, thereby equalizing the inductance value and the coupling coefficient with respect to the primary winding 11. For the above reasons, the switched-mode power supply apparatus being capable of suppressing the difference between the output voltage values even in multi-output form can be obtained.

(2) While in the first and second embodiments the auxiliary windings 14, 16 and control circuit 22 are insulated from the primary winding 11, they may be connected to the primary winding 11, as shown in FIG. 13. Referring to FIG. 13 in connection with the control of the switching element 21, the control circuit 22 has a resistance for dividing the voltage of the auxiliary winding 14, voltage monitoring means for detecting the divided voltage, and IC. The IC calculates the output voltage of the secondary windings 12, 13 based on an input divided voltage value, a ratio of divided voltage, and winding number ratio of the auxiliary winding 14 and the secondary windings 12, 13. The IC generates the control signal such that a predetermined voltage is obtained, and outputs the control signal at the switching element 21.

(3) While in the second embodiment, two or more auxiliary windings 14, 16 are wound around the same winding axis as the windings 11-13 and arranged more away from the primary winding 11 than the secondary windings 12, 13, the above configuration can be reversed. In other words, the auxiliary windings 14, 16 are respectively disposed at both sides of the primary winding 11 and the secondary windings 12, 13 are respectively disposed outside the auxiliary windings 14, 16, in the winding axis direction.

(4) While in the first and second embodiments, the auxiliary winding 14, 16 are arranged on the same line in the winding axis direction of the primary winding 11 such that they do not overlap the secondary windings 12, 13, they may be arranged over and adjacent to the secondary windings 12, 13 such that it has a radius which is different from that of the secondary windings 12, 13 with respect to the winding axis as a center. For example, the auxiliary winding 14 may be arranged outside the secondary winding 13, and the auxiliary winding 16 may be arranged outside the secondary winding 12.

(5) While in the second embodiment, the auxiliary windings 14, 16 are spaced apart from the gap 15 at an equal distance and symmetrically arranged with respect to the gap 15, the invention is not limited thereto. In other words, even in a case where the auxiliary windings 14, 16 are not spaced apart from the gap 15 in an equal distance and not symmetrically arranged with respect to the gap 15, it would be enough that they can evenly exert their action on the secondary windings 12, 13. Furthermore, the auxiliary windings 14, 16 may be arranged closer to the primary winding 11 than the secondary windings 12, 13 in the winding axis direction.

(6) While in the second embodiment two auxiliary winding 14, 16 are provided, three or more auxiliary windings may be provided. For example, in a case where three or more secondary windings are provided, the auxiliary windings may be respectively arranged adjacent to each of the secondary windings, and connected in parallel to each other.

REFERENCE NUMERAL LIST

-   10 core -   10 a center leg portion -   11 primary winding -   12, 13, 17 secondary winding -   14, 16 auxiliary winding -   15 gap -   21 switching element -   22 control circuit -   23, 24 diode (i.e., rectifying means) -   25, 26 capacitor (i.e., smoothing means) -   T transformer -   C line shared by the winding axis of the primary winding and the     winding axis of the secondary winding 

1. A transformer, comprising: a core having a linear center leg portion in a center portion thereof; a primary winding provided around the linear center leg portion of the core; a gap provided in the linear center leg portion of the core at a location where the primary winding faces the linear center leg portion; and at least two secondary windings provided around the linear center leg portion of the core and spaced apart at an equal distance in a winding axis direction of the primary winding from both sides of the primary winding, the at least two secondary windings being spaced apart at an equal distance in the winding axis direction of the primary winding from the gap of the linear center leg portion of the core; and at least one auxiliary winding provided around the core, wherein the secondary windings are disposed closer to the primary winding than the at least one auxiliary winding.
 2. The transformer in accordance with claim 1, wherein the at least one auxiliary winding includes at least two auxiliary windings, and the at least two auxiliary windings respectively neighbor the secondary windings in the winding axis direction of the primary winding, and are connected in parallel to each other.
 3. The transformer in accordance with claim 1, wherein the at least one auxiliary winding includes at least two auxiliary windings.
 4. A switched-mode power supply apparatus, comprising: the transformer in accordance with claim 1; a switching element connected to the primary winding of the transformer; and a control circuit configured to control the switching element.
 5. (canceled)
 6. A switched-mode power supply apparatus, comprising: the transformer in accordance with claim 2; a switching element connected to the primary winding of the transformer; and a control circuit configured to control the switching element.
 7. A switched-mode power supply apparatus, comprising: the transformer in accordance with claim 3; a switching element connected to the primary winding of the transformer; and a control circuit configured to control the switching element.
 8. (canceled)
 9. The transformer in accordance with claim 1, wherein the secondary windings are disposed closer to the primary winding than the at least one auxiliary winding in the winding axis direction of the primary winding. 